Something German Comes My Way

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ERROR EVALUATION In order to guarantee flawless operation and optimum protection of your valuable headphones, the V281 is equipped with various protection circuits: - the delayed activation of the headphone outputs upon power-up prevents your headphones from potentially adverse signal peaks from the preceding circuitry. In a similar manner, the headphone outputs are instantly disconnected when turning power off. - Adverse DC voltage should be kept from your headphones at all times. If DC is detected, the headphone outputs will be disconnected, indicated by the red “MUTE” LED flashing and the green “HEAD” LED lit. The detection interval is about five seconds. If DC disappears during this period, the unit will resume normal operation automatically. - Overloading (i.e. increased distortion) must be avoided in favour of your headphones as well. If overload is detected, the headphone outputs will be disconnected, indicated by the red “MUTE” LED flashing and the green “LINE” LED lit. The detection interval is about five seconds. If overload conditions resolve during this period, the unit will resume normal operation automatically.

FACTORY RESET of all software based settings can be done by pushing the “XLR” button during powering up.

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Things to know …

Why makes it sense to make such huge efforts ? A headphone amplifier is a device designed to condition audio signals with regard to the very specific requirements of headphones. This doesn't sound too spectacular at the first glance and can be achieved relatively easily. As with many things however, the devil is in the details and much more effort is required to design one amplifier for all current headphone models. Headphones per se are quite diverse, and there are two essential parameters: impedance and sensitivity. In general, headphones with higher impedance can be regarded as less sensitive than headphones with low impedance (which is not generally true, but in the majority of cases). The sensitivity of headphones is usually stated in dB (sound pressure level) per Milliwatt. Extremes in this sense are the AKG K1000 with 74dB/mW on the one hand, and the Sennheiser HD25 with108 dB/mW on the other hand: The K1000 requires 2500 times the power to achieve the same sound pressure as the HD25. There is also the fact that headphones with high impedance usually require much higher voltage to achieve high loudness. Thus the amplifier must be designed with high internal supply voltages.

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Which advantages do balanced signals offer ? In contrast to unbalanced signals, balanced signals are carried by two wires (plus ground/shield). In the transmitting device, a balanced signal is created by generating an inverted original signal (180ˆphase shifted). The "hot" wire carries the original signal (a), the "cold" wire the inverted signal (-a). In the receiving device, the balanced signal is processed by a differential amplifier, which detects the difference between both: (a) – (-a) = 2a. On its way between devices, the useful signal can be affected by interference (s). Interferences however are in phase on both wires and fed to the differential amplifier as well. Again, the amplifier detects the
difference between the interference contents: (s) – (s) = 0. Thus - in an ideal situation - all interference on the signal path is eliminated.

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Why are discrete signal paths important ? Twin op-amps are the most common design for operational amplifiers, i.e. two amplifier circuits are integrated in one device. If left- and rightchannel signals are processed simultaneously by such a device, interaction between both cannot be excluded. This interaction is admittedly diminutive, but should be avoided whenever a different design offers the possibility.

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Why are op-amps ideal for low-level signal processing ? Discrete amplifiers (designed with transistors) are very popular in highend audio design also for preamplifier stages. This is often marketed as an optimization measure, but the partially exorbitant extra expenses are of course to be paid by the customer. But an op-amp consists of transistors as well... Moreover, its structure has the advantage of thermal coupling between its internal components. Also ageing issues play a much less important role. Due to the large number of op-amps types offered, it is possible to pick an optimum type for any specific application.

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Why does PRE-GAIN make sense ? Two extreme examples (with the HPA V281 at +14 dB gain (x 4,5), volume control set to full):

1st example: The (pre-) amplifier provides 2V output voltage, whereas the headphone requires only 2V for 100 dB sound pressure level. With the control fully turned up, the V281 would deliver 9 V output at +14 dB gain. Therefore the volume control would have to be operated very carefully in order to avoid hearing damage. Moreover, any interference at the input should be avoided since it would be "unforgivingly" amplified as well. With PRE-GAIN, the input level can be

reduced by 12 dB (a fourth), with 0,5 V instead of 2 V input as the result. This 0,5 V is again amplified by 4.5, then equalling 2.25 V. Now the volume control can be turned over almost the entire range.

2nd example:: The (pre-) amplifier provides 1 V, whereas the headphone requires 20 V to release 120 dB of sound pressure. With the volume control fully clockwise, the V281 would provide 4.5 V at +14 dB gain only - much too low for the headphone. By means of PREGAIN, input level can be boosted by 12 dB (four times), resulting in effective 4 V input voltage instead of 1 V. These are again multiplied by 4.5, now equalling 18 V. This is still not enough, but far closer to the optimum value.

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Why does frequency bandwidth limiting make sense ? In signal processing, sound is represented by AC voltages. Sound is audible - for young people - from about 20 to 20000 Hz. The elder the listener, the less he will hear high frequencies in particular. In order to transmit these frequencies at optimum quality, the frequency response of an amplifier should be as wide and as "flat" as possible. At the low end of the scale, this limit is represented by DC, as there is no frequency lower than zero. In upward direction, the limit can be set to practically any frequency, but the higher, the more susceptible the device becomes concerning electro-magnetic interference. This is not audible in the first place, but may interfere with the useful signal and then become evident. Therefore, unrestricted frequency response attests thoughtlessness rather than remarkable engineering skill.

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Why is a good volume pot essential ? A volume potentiometer is a mechanical control element, which can be obtained on the market at any low price. Meanwhile it is often replaced by electronic circuitry, exhibiting essential disadvantages concerning dynamic range, noise and distortion.
Conductive-plastic resistive tracks, high-quality multi-tap wipers and separated chambers for the individual sections are highly desirable for sophisticated applications, and high quality is inevitable to ensure trouble-free operation for years. Since the market for really good pots is a small one, manufacturers like Noble or Panasonic don't offer these any more. A current sample of top of the line pots is the RK27 by ALPS, which is also used in your HPA V281.

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Why is a low output impedance essential ? When actuated, electro-dynamic systems respond with a counterforce. When the voice coil of a headphone has been displaced by the signal, an (error-) current will be induced when it swings back to its initial position. This current must be suppressed as far as possible, which is effected best if the amplifier's output impedance is the lowest possible. The damping factor describes nothing but the ratio between output impedance of an amplifier and a given load. Since there is no known technical specification, we define the load (voice coil impedance) as 50 ohms. With V281 having an output impedance of <0.2 ohms in balanced mode and <0.1 ohms in unbalanced mode this results in a damping factor of 250 (balanced) and damping factor of 500 (unbalanced).

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Why are high supply voltages essential ? A headphone doesn't really require high power, but from the equation P = U2 / R we can see that the square of the supply voltage determines the power into a given load resistance. The higher the headphone's impedance, the more voltage will be needed. But this deals with the achievable loudness to a limited extent only: Technically spoken, music lives on fast transients which put high demands on signal processing. And thus a fast transient can easily push an average amplifier with +/-15 volts supply to its limits (90 % of all headphone amps in the market are operated with these or even lower supply voltages). Due to the high supply voltage and the balanced operation mode of V281 you will benefit from more than 4 x output voltage swing capability compared to
“standard” amps.